The lower part of the mantle. This is fluid in nature, allowing the plates of lithosphere to “float” on top.
collision zone
A type of convergent margin in which two continents or island arcs have collided. These zones occur at convergent plate boundaries involving continental crust. Continental crust lacks the density of oceanic crust and, as a result, does not sink into the mantle. When continental crust collides with continental crust the rocks are compressed and deformed through folding and faulting. This process was responsible for the formation of the Himalayas as the Indian plate moved into the Eurasian plate.
continental
drift hypothesis
The hypothesis, proposed by Wegener, that the continents had once been joined in a supercontinent called Pangaea. This is supported by fossil evidence and the way the present-day continents appear to fit together.
convection currents
The lower part of the mantle (the asthenosphere) is heated by the Earth’s core. The heated mantle rises to the surface where it cools and sinks. The current caused by this process influences the motion of the lithospheric plates.
convergent plate boundaries
The boundaries where two lithospheric plates are moving towards each other.
This often results in collision zones and subduction zones.
crust
The thin surface layer of the Earth surrounding the mantle. There are two types of crust: continental and oceanic. Continental crust is thicker and less dense than the oceanic crust.
divergent plate boundaries
The boundaries where two lithospheric plates are moving away from each other. This often results in sea-floor spreading and rifts.
earthquake
A sudden movement of the Earth’s crust caused by seismic activity occurring due to a release of pressure and movement in the lithosphere. Due to the interaction between lithospheric plates, earthquakes are common along plate boundaries.
fault
A fracture in the crust where there is movement relative to the opposing block. There are three main types of faults: normal, strike slip and thrust.
Gondwana
One of the two supercontinents formed at the separation of Pangaea. It comprised the present-day continents of Antarctica, South America, Africa, Australia, New Zealand, the Arabian Peninsula and the Indian subcontinent.
inner core
The centre of the Earth, which is the hottest part.
Laurasia
One of the two supercontinents formed at the separation of Pangaea. It comprised the present-day continents of North America, Europe and Asia (excluding India).
lithosphere
The rigid layer formed by the upper mantle and the crust.
mantle
The viscous layer of the Earth under the crust. The upper part of the mantle is less fluid and, along with the crust, comprises the lithosphere. The lower mantle is heated by the core, causing convection currents to occur.
mid-ocean ridge
The result of a divergent plate boundary. Mid-ocean ridges are formed when the plates move apart (sea-floor spreading) and magma rises to the surface, forming new rocks in the form of undersea mountains.
outer core
The liquid layer of the Earth located between the solid inner core and the mantle.
Pangaea
A supercontinent containing the entire land mass of the Earth. The initial split in this supercontinent resulted in Laurasia and Gondwana being formed.
plate
A section of the lithosphere that moves over the asthenosphere.
plate boundary
The edges where lithospheric plates meet each other. There are three boundary types: convergent, divergent and transform.
plate tectonic theory
The theory based on the continental drift hypothesis. It draws upon geological evidence (such as the “jigsaw puzzle”), fossil evidence and paleomagnetic data to explain how lithospheric plates move over time. This theory has been further supported by evidence based on technological advances.
relative plate motion
The motion of a lithospheric plate in respect to a neighbouring one.
rift valleys
A portion of the earth's crust, bounded on at least two sides by faults, that has been moved downwards in relation to the adjacent portions. These are formed as the boundaries of lithospheric plate move in opposing directions. In plates where the rift occurs in oceanic crust, sea-floor spreading occurs, leading to a rift valley associated with the creation of new crust. When the rift occurs in a continental plate, a valley forms. This can lead to the separation of the plates either in part or completely.
sea-floor spreading
A process that occurs at mid-ocean ridges, where new oceanic crust is formed through volcanic activity and then gradually moves away from the ridge.
subduction zone
An area of convergence between a sinking tectonic plate and an overriding plate. These zones occur at convergent plate boundaries where oceanic crust is involved. Oceanic crust is dense and is pushed (subducted) into the mantle. Where oceanic crust meets continental crust, the thicker (and less dense) continental crust overrides the oceanic crust. When two plates with oceanic crust meet, one plate is usually of higher density and, therefore, is subducted under the other.
transform plate boundaries
The boundaries where two lithospheric plates slide past each other. The release of stress often causes shallow earthquakes to occur, although there is generally no volcanic activity associated with these boundaries. They usually occur in oceanic crust, although the San Andreas fault, found in continental parts of the North American plate, is an exception. These are also referred to as “transform faults”.
volcano
An opening in the Earth’s crust formed when magma rises to the surface of the Earth and escapes through an opening in the crust. Most volcanic activity is associated with the interactions that occur at plate boundaries. They are typically found at convergent boundaries where subduction occurs, and at divergent boundaries.
drift hypothesis
This often results in collision zones and subduction zones.